Inverse Multiplexing (IM) is a data communication technique that allows a grouping of lower speed communication links into one logical higher speed communication link of approximately the same transmission bandwidth capacity as the sum of the lower speed communication links. Such a grouping of lower speed communication links for facilitating IM are referred to herein as a group of IM communication links. IM techniques are often used when cost or technical feasibility prevents deployment of a single higher speed point-to-point communication link. For all essential purposes, the group of IM communication links behaves identically to a single point-to-point higher speed communication link of the same capacity.
One example of an application for utilizing IM techniques is to increase the speed of Internet access beyond what is capable with a single xDSL (e.g. SDSL, ADSL, etc.) service. This is particularly important to subscribers living further from the central office communication apparatus providing the subscriber's service because xDSL link rates reduce with increasing distance. Another example of an application for utilizing IM techniques is to deliver basic video communications using xDSL services. The majority of xDSL subscribers are capable of receiving xDSL services in the speed range (i.e. bit rate) of at least about 1.5 Mbit/s to about 2 Mbit/s. Such a bit rate is generally not considered sufficient for the delivery of entertainment grade video and interactive video services. However, by combining two or more IM communication links into one logical link, such entertainment grade video and interactive video services may be facilitated for most xDSL subscribers.
Asynchronous Transfer Mode (ATM) is a transport protocol that is widely deployed in high-speed data networks and that allows the multiplexing of different information streams across one ATM formatted communication link. IM over ATM (IMA) related to an ATM Forum standard (i.e. AF-PHY-0086.001) which outlines a standardized technique for using IM techniques over ATM formatted communication links. In the case of ATM, a group of ATM formatted IM communication links (i.e. an IMA group) behaves as if it were a single higher speed ATM communication link. For example, in the case of IMA over a pair of IM communication links each having an upstream data transmission rate of 1.5 megabits and a downstream data transmission rate of 1.5 megabits, the aggregate data transmission rate would be 3.0 megabits in both the upstream and the downstream directions.
In order to facilitate the recombination of the traffic that is distributed across the individual links in a group of IM communication links, the IMA standard of the ATM Forum assumes that each IM communication link in an IMA group operates at exactly the same bit rate and with a constant delay. Resequencing at a receiving end of the IM communication links is then a simple matter of reordering ATM cells based on a function of the arrival time and link delay. The differential delay between any two IM communication links within a IMA group is required to remain less than about 25 ms. Some ATM transport technologies (particularly some of the modes supported by some of the xDSL technologies) may lead to differential delays exceeding this value under some circumstances, therefore preventing the establishment of an IMA group or preventing the addition of some additional IM communication links into an existing IMA group.
In order to manage the individual links within an IMA group, the ATM Forum IMA standard requires the insertion of special purpose OAM (Operations and Maintenance) cells into each of the links comprising an IMA group. Depending on the configuration, these management cells consume either 1/32, 1/64, 1/128 or 1/256 of the available capacity which is therefore unavailable for carrying subscriber traffic.
Conventional IMA (e.g. the ATM Forum IMA standard) implementations are capable of being facilitated over symmetric transmission facilities and uniform transmission facilities. A symmetric transmission facility includes a plurality of IM communication links each have the same data transmission rate in an upstream and a down stream direction. A uniform transmission facility includes a plurality of IM communication links each having the same upstream data transmission rate and each having the same downstream data transmission rate, wherein the downstream data transmission rate may be different than the upstream data transmission rate.
A key limitation of conventional IMA implementations is that such implementations are not capable of being facilitated over a plurality of communication links having disparate data transmission rates. Conventional IMA implementations require that each one of the IM communication links in an IMA group have a common upstream data transmission rate and a common downstream data transmission rate. For example, all of the downstream data transmission rates being 1.5 megabits and all of the upstream data transmission rates being 500 megabits. This requirement limits the utility of IMA because the upstream data transmission rates, downstream data transmission rates or both are often different for different communication links.
Multiple ADSL communication links often do not synchronize at the same data transmission rates. Furthermore, ADSL links are inherently rate adaptive. This means that the communication device (e.g. line card, modem, etc.) on each end of an ADSL connection commonly negotiate and attempt to establish the highest bit rate permitted by their configuration. If during the life of the link, conditions change sufficiently as to warrant an increase or decrease in link speed, the communication devices renegotiate a new link speed automatically.
When selecting two or more ADSL links for bonding into an IMA group, there is a significant probability that the optimum speed for each link will differ. Therefore, in order to implement the ATM Forum standard for Inverse Multiplexing, the faster links must be slowed down to the same rate as the slowest link in the IMA group. This is non-optimal because potential capacity (ATM throughput) is foregone in satisfying the criteria that IM communication links in an IMA group operate at the same data transmission rate.
Once such an IMA group is established, should any of the links in the IMA group experience degraded line conditions necessitating a further reduction in a respective bit rate, all of the IM communication links in the IMA group must be renegotiated to the lower bit rate in order for the IMA group to remain operational. This results in further foregone capacity and a short disruption in service for the subscriber as each of the links and then the IMA protocol resynchronize. Similarly, if conditions improve warranting an increase in capacity on all IM communication links, there is likewise a short service disruption for the same reason.
The operation of the ATM Forum IMA protocol during establishment, operation and decommissioning of an IMA group is relatively complex and requires specialized hardware and/or software to support the protocol. The complexity adds a significant cost to equipment using known implementation techniques. The added cost is enough in cost sensitive applications as to make utilization of IMA using conventional IMA implementations difficult to justify.
Therefore, facilitating IMA via IM communication links synchronized at disparate data transmission rates in a manner than overcomes limitations associated with facilitating IMA via IM communication links synchronized at a common data transmission rates is useful.